A public utility, such as an electric generating company, has certain public responsibilities and obligations. One of the responsibilities of an electric utility is to ensure sufficient electricity for the needs of residential, commercial and industrial users. Electric utilities routinely forecast future needs and build generating facilities to accommodate these needs.
For a long period of time, electric utilities were able to predict growth without very much difficulty. However, some years ago the growth pattern of electricity use changed. For example, for many utilities located in temperate climates, the demand for electricity during the times of the year when air conditioning is used is far greater than the demand for electricity during the seasons when air conditioning is not in widespread use. Simply put, the demand for electricity in the summertime is far greater than the demand for electricity in the wintertime for these utilities. For utilities in colder climates, electrical usage may peak in the wintertime. To carry out its public charge, however, electric utilities must plan sufficient production facilities to meet the highest anticipated demand during the year.
There are several ways that electric utilities meet the peak electricity demand. One approach is to use pumped storage facilities which pump water into a reservoir during off-peak periods and then use the stored water to run a hydrogenerator during peak periods of the day. It is known that in the summertime, the peak period will probably occur between twelve noon and midnight. With this information, an electric utility can plan to use electricity to pump water into the reservoir between midnight and noon, and then use the stored water between noon and midnight to run hydrogenerators to meet the peak demand. Another method of meeting the peak demand is to utilize an additional oil or coal fired generating unit. Yet another method is to purchase the electricity from another utility which has excess capacity, albeit at a premium. All of these methods, needless to say, are expensive propositions for the utility.
While it appears that the electric utilities are in a difficult position, it is the electric utility customers who are ultimately responsible for the bill. It is arguably fair that the users of electricity who create the demand for extra facilities or for expensive electricity purchased from other utilities should pay extra for having their demand met. This is exactly what the electric utilities attempt to do. For residential customers, for example, the electric utilities may stagger the rates so that for a base amount of electricity use, the residential customer pays a small amount, and for each successive unit above the base amount, the residential customer pays based upon a rate which gradually increases. This staggered rate is designed to encourage residential customers to conserve electricity.
Similarly, commercial and industrial users are also encouraged to conserve electric energy. With large commercial and industrial users, measurement of the electric energy consumption is somewhat different than with residential customers. Residential customers typically pay for each kilowatt-hour (kWh) of electricity used. On the other hand, commercial and industrial users have their energy consumption measured not only in terms of kWh used, but also by a demand meter which functions to record the peak usage of that particular customer. With this system, the customer can be charged a rate or premium according to his peak usage, since it is this peak usage to which the electric utility must build facilities to generate electricity. Accordingly, it behooves these customers to keep their peak usage as low as possible.
Energy management systems ("EMS") came into being to assist heavy electric users reduce overall consumption and keep peak energy usage below a predetermined level. Electric utility companies have monitored electric consumption for years and have systematically dropped load from selected customers to prevent the energy demand from exceeding the energy supply. Electric utilities were able to do this by identifying certain users of electric energy who could tolerate temporary interruptions of power without great harm. Energy management systems allow smaller commercial or consumer users to do the same thing. For example, there are known hotel/motel energy management systems which selectively turn off air conditioners when a room is unoccupied or unrented, or periodically interrupt the air conditioner operation during peak load times.
U.S. Pat. No. 4,059,747 discloses a demand controller which controls the operation of one or more interruptible electrical loads to maintain the total electrical power demand during a demand interval within certain predetermined limits. The demand forecast makes periodic projections of the metered demand to the end of the demand interval based upon the current rate of energy consumption ("instantaneous demand") and the accumulated energy consumption during the demand interval. If the projected value indicates that a demand peak will occur, the system signals a "shed" condition for the load, thereby interrupting the load to prevent the occurrence of the peak. Conversely, if the system detects a projected low value of the demand, a "restore" condition is triggered, whereby the load is turned on to take advantage of the available electrical capacity.
U.S. Pat. No. 4,489,386 discloses a device control system which provides a method of determining when loads are to be disabled, to prevent the total consumption from exceeding a predetermined threshold. This system bases the decision-making process on past consumption, past load shed and activity, and predicted future load activity. The controlling algorithm is applied to the present consuming situation for a plurality of different time interval configurations, and the worst-case result of these computations controls the load shedding.
While energy management systems are satisfactory for keeping energy consumption below a preselected threshold, it is apparent that energy usage could be further reduced by appropriate planning. For example, in a hotel or motel, energy usage could be reduced by operating water heaters between midnight and the early morning hours when other energy usage is low and operating the heating/air conditioning only during preselected times regardless of what a room occupant does with the thermostat. While it is conceivable that such a scheme could produce some results, the consequence of exceeding the preselected peak demand is a severe penalty from the power company. Understandably, penalties for exceeding the peak demand are to be avoided. Accordingly, it will be understood that it would be highly desirable to provide an energy management system which not only maintains the energy usage below a predetermined threshold, but which also gives an indication as to the amount of energy management system is saving.